I think the part that is missing for me is the context, in other words how would one configure the execution_plan and why would they configure it in a specific way?
My intuition, without fully understanding it, is that for some reason the internal DAG/decision is exposed to the user, and it feels like too much information. Basically I have a hunch that the users should not need to have such deep understanding to control the flow, and they should end up with an abstraction on top of it. Does that make sense ?